Generally, an aircraft has one or more primary engines that provide thrust for the aircraft and pressurized bleed air for the environmental control systems. The primary engine also provides power to drive electric generators and hydraulic pumps that are necessary for powering instruments and flight control systems. In turbine-powered aircraft, aircraft control surfaces generally are linked by electrical or hydraulic circuits. Consequently, in the event of an electrical or hydraulic failure, the pilot cannot alter the aerodynamic configuration of the aircraft until power is restored. As a result, turbine-powered aircraft require an emergency power unit that is capable of responding to a power failure and providing a sizable quantity of electrical or hydraulic energy rapidly such that the pilot can regain control of the aircraft.
Turbine-powered aircraft, like other complex aircraft, also require an auxiliary power unit to provide electrical and hydraulic energy, and bleed air, when the primary engine or engines of the aircraft are not in use, for example when the aircraft is on the ground. The auxiliary power unit also can provide power to start the primary engines, either on the ground or in flight. Both the primary engine and the auxiliary power unit operate on aviation fuel drawn from the main fuel tanks of the aircraft. Combustion is supported by mixing the aviation fuel with air drawn from the atmosphere. In many instances, starting the auxiliary power unit requires an external power source such as a battery, a pressurized air tank or a hydraulic accumulator. Typically, both the emergency power unit and the auxiliary power unit can be employed to drive a generator and/or a hydraulic pump.
In some aircraft, an auxiliary power unit easily can be adapted to serve as an emergency power unit as well, thereby minimizing space and weight requirements. However, the adaptation is difficult on high performance aircraft that operate at high altitudes. In particular, a typical auxiliary power unit is an air-breathing engine designed primarily to operate on the ground where the air is relatively dense. Therefore, the auxiliary power unit may be incapable of operating at high altitudes, for example above 55,000 feet, where the density of the air is insufficient to start the turbine and rapidly bring the turbine up to an operating speed that produces emergency power in an emergency situation. It is therefore evident that in many situations the auxiliary power unit could not restart a failed primary engine at a high altitude, and accordingly no electrical or hydraulic power would be available for control of the aircraft.
It is necessary therefore to equip an aircraft with an auxiliary or an emergency power unit that is capable of operating independent of external conditions, like air density; that can provide in-flight emergency electrical and hydraulic power to the flight control systems; and that can be used to restart the primary engines in-flight. Since the emergency power unit is operated only in the event of an emergency, the unit remains stored and inactive for long periods of time, but is required to start quickly and to provide continuous power output for a prespecified duty cycle. Ideally, such an emergency power unit is compact, lightweight, reliable, easily maintained, and requires no special handling of materials or fuels, while providing a combustion process that is controllable and that produces a clean, nontoxic combustion gas.
To overcome some of the above-discussed problems, Friedrich, in U.S. Pat. No. 4,092,824, disclosed a turbine used for starting aircraft engines and for driving auxiliary equipment, such as a generator. The turbine is capable of operating in a conventional air-breathing mode as well as in an emergency mode that does not require air. In particular, Friedrich includes a supply of hydrazine on the aircraft. Hydrazine is capable of undergoing an exothermic decomposition reaction, and the heat of decomposition is utilized to vaporize aircraft fuel and thereby provide a volume of hot gas that drives the turbine in an emergency situation.
The method disclosed by Friedrich solves a number of the previously-specified problems, but the method also has definite disadvantages. Liquid hydrazine is corrosive and toxic, thereby requiring special handling procedures and design considerations. In addition, the soot-like decomposition products of hydrazine method can accumulate within the turbine and decrease turbine efficiency. More significantly, because the method is based on an exothermic decomposition reaction, it necessarily follows that a fuel utilized in the method, like hydrazine, must be sufficiently unstable to undergo a rapid decomposition. The presence of such an unstable fuel on an aircraft presents unacceptable hazards. Still another disadvantage is that hydrazine and proper hydrazine storage facilities may not be available at all ground support locations for the aircraft. Thus, recharging an aircraft with hydrazine fuel becomes a problem, particularly because hydrazine is toxic and is difficult to handle.
To avoid the problems and disadvantages inherent in a hydrazine-based system, oxygen preferably is utilized as the oxidant for fuel in an aircraft emergency situation. The oxygen supply can be in the form of compressed air stored in tanks on the aircraft. However, compressed air has the disadvantages of requiring extensive, and expensive, landbased recharging stations and of including a relatively low concentration, i.e. 21%, of oxygen.
Therefore, it has been proposed that oxygen-enriched air or pure oxygen be used as the oxidant for fuel in an aircraft emergency situation. Oxygen or oxygen-enriched air can start a turbine more easily when combusted with fuel and can bring the turbine up to speed more reliably than compressed air. Therefore, if oxygen or oxygen-enriched air is utilized as the fuel oxidant, the difficulty in reliably bringing a turbine up to speed to provide power at high altitude is overcome. Additionally, neither oxygen nor oxygen-enriched air is unstable in the same sense as hydrazine. However, even though oxygen and oxygen-enriched air overcome many of the problems presented by hydrazine and compressed air, it would be desirable to eliminate the need for extensive landbased recharging or regenerating stations and to reduce the space and weight requirements of extensive on-board storage of oxygen or oxygen-enriched air.
Ambient air contains a sufficient amount of oxygen to oxidize a fuel in an emergency situation. But the oxygen must be isolated from the ambient air in a substantially pure form and stored for later use. Investigators therefore sought methods of isolating oxygen from the ambient air to meet the oxygen quality and quantity requirements with a compact and lightweight apparatus. Consequently, numerous methods and systems were devised to separate a stream of air having an ambient concentration of oxygen and nitrogen into a usable supply of gas possessing an enhanced oxygen concentration and into a usable supply of gas possessing an enhanced nitrogen concentration.
For example, Manatt in U.S. Pat. No. 4,508,548 disclosed an air separation module based upon the differing permeabilities of oxygen gas and nitrogen gas through a hollow, permeable film. The method utilizes a pressure gradient to separate the oxygen from the nitrogen in air. However, the method provides only moderately oxygen-enriched air, i.e. 35-45% oxygen, whereas ambient air includes about 21% oxygen; and provides nitrogen-enriched air still containing about 9% oxygen. Manatt teaches that such oxygen-enriched air can be used in aircraft for breathing purposes, but no suggestion was made that such moderately oxygen-enriched air is suitable as a fuel oxidant in an aircraft auxiliary or emergency power unit. In contrast, the oxygen separation system and method of the present invention provide a substantially pure stream of compressed oxygen gas including about 99% or greater oxygen.
Wiegand et al. in U.S. Pat. No. 4,777,793 disclosed an aircraft emergency power unit that utilizes compressed air as the fuel oxidant. The compressed air can be stored in tanks or can be the bleed air from the main aircraft engines. Wiegand et al. do not teach or suggest separating the compressed air to provide an oxygen-enriched gas stream.
Vershure in U.S. Pat. No. 4,827,716 disclosed that oxygen-enriched air is preferred over compressed air in an aircraft emergency situation, such as in restarting a primary engine at a high altitude. Vershure teaches the separation of bleed air into an oxygen-enriched air stream and a nitrogen-enriched air stream, followed by compression and storage of the gas streams for future use on the aircraft. However, Vershure does not teach or suggest a particular method or apparatus for separating the bleed air into enriched gas streams.
Fee et al. in U.S. Pat. No. 4,877,506 disclosed a ceramic oxygen separator having a particular corrugated configuration. Fee et al. do not teach or suggest the use of a ceramic separator on an aircraft to provide compressed oxygen-enriched and nitrogen-enriched gas streams.
Another type of oxygen generator is discussed in Aviation Week and Space Technology, pp. 56-57, (Feb. 3, 1980). This publication described an adsorption-desorption interaction between oxygen and a molecular sieve to provide breathing oxygen for an aircraft crew. The method utilizes a molecular sieve, such as a zeolite, to preferentially adsorb the oxygen in the bleed air over the nitrogen, and therefore store the oxygen for later use.
Therefore, in summary, emergency power units have been developed for quick high altitude starts of a primary aircraft engine or for other uses. Often, compressed air is used as the fuel oxidizer in starting the emergency power unit. However, the reliability of the start is increased significantly if compressed oxygen-enriched air is used as the oxidizer rather than compressed ambient air. Furthermore, as the percentage of oxygen in the compressed gaseous stream increases, the reliability of the start increases. Additionally, equipment currently used for storing compressed air or oxygen-enriched air is heavy and requires regular replenishment from landbased facilities.
The prior methods and systems utilized to provide a sufficient amount of oxygen on an aircraft to quickly and reliably start and maintain combustion in an emergency power unit included tanks of compressed air or of oxygen. This method has the disadvantages of requiring storage tanks that are heavy and bulky, and depending upon landbased recharging stations.
Generating oxygen or oxygen-enriched air on board on aircraft, and in-flight, eliminates the need for recharging facilities at each airbase. But, until the system and method of the present invention, the aircraft also required an on-board compressor to compress the oxygen or oxygen-enriched gas stream. Therefore, in accordance with an important feature of the present invention, a substantially pure stream of oxygen gas, including about 99% or greater oxygen, is generated in-flight from an atmospheric air source, such as bleed air, ram air or unpressurized ambient air. The method selectively separates the oxygen from the air source, and the generated stream of substantially pure oxygen can demonstrate a pressure up to about 10,000 psi, thereby eliminating the need for an on-board compressor to compress the substantially pure stream of oxygen.